1. Field of the Invention
This invention relates to an amorphous silicon hydride photoreceptor for electrophotography which causes no "image blurring" (as defined hereinafter) in use under a high-humidity environment, a process for the preparation and/or regeneration thereof, and a method of electrophotography using such a photoreceptor.
2. Description of the Prior Art
Electrophotography is but one of several permanent image forming methods, the principle of which was discovered by Carlson in 1938. Various improvements have been made on the technique and now electrophotography is widely applied in the image recording arts, for example, in plain paper copiers (PPCs), laser printers, facsimiles" and the like: see R. M. Schaffert, "Electrophotography", 1975, Focal Press Ltd.
Generally, an electrophotographic method comprises several elementary processes. In PPCs, for example, the method usually comprises the following processes:
(1) Charging
Corona ions of a certain polarity (for example, the positive polarity) produced by a corona charger are accumulated on the surface of a photoreceptor in the dark. Since the photoreceptor is electrically an insulator having resistivity (p) above 10.sup.12 ohm-cm in the dark, the surface of the material is positively (or otherwise negatively) charged and the acceptance potential usually reaches about 400 V or higher.
(2) Exposure
Light (usually visible ray) is irradiated to an original so that an image of light reflected from the original is formed on the surface of the photoreceptor by means of an appropriate optical lens system. Since the irradiated part of the material becomes electrically conductive, the charge on this part is leaked away to the earth. Thus, an image of charge corresponding to the original image to be copied is produced on the surface of the photoreceptor. The image of charge is called an electrostatic latent image.
(3) Development
Development may be carried out by either the wet or dry process. In recent years the dry process has become more popular due in part to their being no need for any special solvent. The dry process is further divided into a dual-component system and a mono-component system. The dual-component developing system, there are used.
In the dual-component system, there are used resin particles containing a coloring material, namely toner, such as carbon black and magnetic particles, called a carrier. The toner and carrier are mechanically mixed so that they may be charged due to the triboelectric effect. The polarity of the charge may be controlled by the type and amount of a charge control agent, such as a dye and a pigment, in the toner. When the photoreceptor has a positively charged electrostatic latent image, a toner will have the opposite polarity; that is, a negatively charged toner is used.
The toner and carrier are distributed on the surface of a cylinder, called the "Magroll," which incorporates a magnet and is designed to leak out a magnetic flux. Thus, the carrier, which is a magnetic substance, and the electrostatically attracted toner are arranged along the magnetic flux on the Magroll surface, and a so-called "ear" is produced. Generally, the Magroll is rotated and the ear is contacted with the photoreceptor. The toner is attracted to the opposite, positive charges on the photoreceptor and moves from the carrier to the surface of the receptor material.
Consequently, a visible toner image corresponding to the electrostatic latent image is formed on the photoreceptor. This process is generally known as the magnetic brush development.
The toner which moves from the magnetic brush to the photoreceptor and is consumed in the developing process can be properly supplied in the course of the process, so that the mixing ratio of the toner and the carrier may be maintained at a constant value; usual weight ratios of the toner to the carrier are in the range of about 2% to 6%.
(4) Transferring
The visible toner image on the photoreceptor is then transferred onto plain paper. This is accomplished by placing the paper on the surface of the photoreceptor and applying corona discharge, which has a polarity (e.g., positive as in the above case) opposite to the toner charger polarity (e.g., negative as in the above case), to the paper from its back, i.e. the side opposite to the side thereof contacted with the photoreceptor. Thus, the toner is attracted and transferred from the photoreceptor to the paper surface.
After the transferring, the paper is fixed, whereas the photoreceptor is discharged, cleaned and charged again for repeated use in the foregoing process steps (1) to (4). The fixing, discharging and cleaning processes are carried out in the following manner.
(5) Fixing
The toner on the plain paper is heated above the softening point of a resin which is contained as a main component in the toner, so that a semipermanent visible image may be obtained. Generally, the paper and toner are pressed by heated rollers or passed over an electrical heater (e.g., an oven).
(6) Discharging
In this process, the electrostatic latent image on the surface of the photoreceptor is eliminated. Two methods are usually used. One is the photo-discharging method in which light is irradiated on the whole surface of the photoreceptor to remove the surface charge by the photoconductivity of the material. The other is the method in which an alternating electric field is applied to a corona discharger so that alternating charged particles (corona with either positive or negative polarity) may be alternatively charged to the surface of the photoreceptor, resulting in the neutralization of charges on the surface of the material. These two methods may be used alone or in combination.
(7) Cleaning
A part of the visible image toner still remains on the surface of the photoreceptor after the development, transfer and discharging processes. The remaining toner is eliminated from the photoreceptor in a cleaning process. Cleaning is generally carried out mechanically. In the rotating brush method, the toner on a photoreceptor is removed by a rotating brush made of a resin such as nylon. In the blade cleaning method, the toner is scraped off by contacting the edge of a blade made of silicone rubber or the like with the surface of a photoreceptor.
The cleaned photoreceptors may be again subjected to the foregoing processes (1) to (7). Such a photoreceptor is usually in the form of a drum and passed through said series of processes while rotating. A corona charger, an optical exposure system, a developing unit, a transfer charger, a corona discharger and a cleaning unit are disposed so as to surround the photoreceptor drum.
Photoreceptors which have hitherto been used for electrophotography include dispersions of fine CdS, ZnO or other powder coated with the binder of organic material, Se (selenium) photoreceptors containing As or Te coated by the vacuum evaporation method, organic photoconductor and trinitrofluorene, and others.
More recently, attention has been drawn to the semiconductive properties, especially photoconductivity, of amorphous silicon hydride (hereinafter referred to as "a-Si") in the form of a film made by the plasma chemical vapor deposition (P-CVD) from silane (Si.sub.n H.sub.2n+2) gas, and studies have been made on the use of a-Si as photoreceptors for electrophotography as well as on the applications to solar cells, photosensors, imaging tubes, and the like.
Photosensitive a-Si films are made from raw materials containing essentially of silane gas by plasma CVD, as mentioned above. Various proposals and improvements have been made on the items required as a photoreceptor, such as acceptance voltage, spectral sensitivity and photoreceptor life.
For example, addition of diborane, ammonia, oxygen, hydrocarbon, germane (Ge.sub.n H.sub.2n+2), fluorosilane, and the like, to the raw material silane gas, and improvement of layer structures on a conductive substrate have been attempted. The photoreceptors based on such improved silicon hydride will hereinafter be referred to as "a-Si photoreceptors".
The a-Si has the following excellent properties as compared with the aforementioned conventional photoreceptors:
(i) It is innoxious and causes no pollution problem; PA0 (ii) It is thermally stable (its crystallization temperature is 400.degree. C. or higher); that is, its thermal stability is markedly higher than those of Se (crystallization temperature; 60.degree. C.), organic substance-based photoreceptors, OPCs, ZnO or CdS; PA0 (iii) It shows higher photosensitivity, and the sensitivity is substantially constant over the wider range of visible light wave length; and PA0 (iv) It has high surface hardness (Vickers hardness of 1,500 or higher) and is tough against the surface damage such as scratching. PA0 (A) Incorporation of a substance excellent in the polishing propery as an additive (external additive) to the developer; PA0 (B) Addition of a process for polishing the a-Si surface, independent of the developing process; and PA0 (C) Improvement of a member for cleaning so as to attain the addition of the polishing effect (B). PA0 (i) the mean surface roughness along the center line as measured by a needle type surface roughness tester being 190 Angstroms or less; PA0 (ii) the mean surface roughness along the center line as measured by a coordinates measuring scanning electron microscope and a section measuring apparatus being 60 Angstroms or less; PA0 (iii) the variance of mean surface roughnesses along the center line as measured by a coordinates measuring scanning electron microscope and a section measuring apparatus being 70 Angstroms or less; PA0 (iv) the maximum surface amplitude as measured by a coordinates measuring scanning electron microscope and a section measuring apparatus being 450 Angstroms or less; and PA0 (v) the difference between the mean of five largest values of the surface roughness as measured by a coordinates measuring scanning electron microscope and a section measuring apparatus and the mean of five smallest values of said surface roughness being 420 Angstroms or less.
For making best use of such advantageous properties, attempts have been made to apply the a-Si photoreceptors to high-speed PPCs, laser printers, facsimiles, and the like, as high-performance photoreceptors for electrophotography. However, the following problems have been pointed out.
Since a-Si photoreceptors have such chemical and thermal stability and high surface hardness as mentioned above, they were at first expected to exhibit both stable performance in use as photoreceptors under various environmental conditions and to endure a very large number of copying procedures. It was acutally found, however, that a phenomenon could occur in which the copy image is blurred with electrophotographic copying procedures using such a-Si photoreceptors were carried out under high-humidity conditions, more specifically, under a relative humidity (RH) above about 70%, and furthermore, no image could be obtained at all under higher humidity conditions: this phenomenon is herein referred to as "image blurring".
This image blurring becomes more conspicuous as copying procedures are repeated. Usually, the image blurring occurs when the charging-exposure-development-transfer process is repeated several thousand times in an ordinary PPC (plain paper copier) under a high-humidity condition. However, the reversibility of this phenomenon has also been found: that is, a clear image can be obtained if copying is again conducted under a low humidity after the image blurring has occurred under high humidity.
It may be considered that such image blurring can be prevented by continuously heating an a-Si photoreceptor drum at a temperature of from about 40.degree. to 50.degree. C. so that the relative humidity around the drum surface may always be maintained below about 50%, even if the environmental humidity varies. In this case, however, an increased cost of the copying machine and its more complicated mechanism are unavoidable because of the incorporation of a heater into the drum and the provision of a temperature controller.
Thus, the image blurring is characterized by (1) the tendency to occur under high humidity, and (2) the tendency to be more conspicuous as copying procedures are repeated.
The cause of the image blurring has not yet been completely elucidated. Such image blurring will not be a serious problem for conventional photoreceptors, for example, those utilizing Se photoreceptors. In their efforts at overcoming this image blurring problem, the present inventors closely studied the above characteristics, especially the latter, and as a result of these studies, formulated the present invention.
The present inventors found in the course of their experiments the following two facts which appeared to give a solution to the image blurring.
(I) The primary cause of image blurring which become conspicuous with repeated copying procedures is the deterioration of the surface of photoreceptors due to corona charge. This fact was revealed in the course of the following experiments on each elementary process of electrophotography, i.e. corona charging, exposure, development or cleaning.
A commercially available copying machine was modified so that only one of the elementary processes, that is, (a) the initial corona charging and the alternative corona discharging, (b) the light irradiation, (c) the magnetic brush development, (d) the cleaning, might be conducted separately, and each operation was repeated as many times as the number of operating procedures corresponding to that required to take 10,000 copies. Thereafter, a copying test was carried out at 30.degree. C., 80% RH (relative humidity) to examine the occurrence of image blurring. The phenomena most often occurred with a-Si photoreceptors which had been subjected to the repeated corona charge (a) alone.
On the other hand, a commercially available Se photoreceptor was treated in the same manner by the repeated corona charge alone at 30.degree. C., 80% RH. Similar image blurring was observed.
Thus, although image blurring also occurs in conventional Se photoreceptors treated by corona charge alone, it may be supposed that the surface layer of the conventional materials, which have a relatively low surface hardness, is gradually scraped off in the course of the entire processes in PPC, in particular the development and cleaning processes and, therefore, a fresh surface condition may always be maintained.
On the contrary, a-Si photoreceptors have extremely high surface hardness. Accordingly, it may supposed that when a deteriorated surface layer is produced by corona charge, the deteriorated layer is not thoroughly removed even through the magnetic brush development and/or cleaning process, but rather such a layer will be accumulated as copying procedures are repeated.
This is apparently the reason why such image blurring, which is not significant with conventional photoreceptors, causes a serious problem in the electrophotography using a-Si material.
(II) As described in our copending Japanese Patent Application No. 228252/84, ESCA (Electron Spectroscopy for Chemical Analysis) of the outermost surface of an a-Si photoreceptor having been subjected to corona charge revealed that an oxide layer was formed to a depth of approximately 100 Angstroms from the surface layer. Thus, it is evident that such a surface layer has a degree of oxidation distinctly different from that of a surface which has not been subjected to corona charge.
The present inventors have discovered that the problem of image blurring in a-Si photoreceptors can be solved by either improving one or several processes in the electrophotographic operation or by adding an additional process to the conventional electrophotographic processes, whereby to effect gradual removal (i.e., polishing) of the surface of the a-Si photoreceptors.
In order to attain the solution there will be the following three means:
Such means have already been proposed for use in conventional electrophotography and are in part disclosed in one or more conventional electrophotographic patents. For example, the use of common hard abrasive substances such as CeO.sub.2, BaTiO.sub.3, SiC and Al.sub.2 O.sub.3 as an external additive for a developer has been attempted. It has also been proposed that a cleaning blade may be made of a material having a higher surface hardness than a-Si.
According to the investigations by the present inventors, however, it has been found that the conventional, hard abrasive polishing of the surface cannot give any satisfactory solution to the image blurring if removal of the deteriorated surface layer of a-Si materials is attempted. The present inventors have discovered that in order to obtain the desired polishing effect in a-Si photoreceptors, a novel abrasive, having a surface hardness lower than that of a-Si and being solid-phase reactive therewith, must be used.